The present invention is in the general field of perforated plastic film and especially relates to vacuum perforating of plastic film. The invention particularly relates to metal screens or molding elements used in the vacuum perforation of plastic film and to a method of fabricating such screens.
Perforated plastic film has many useful applications. It is used in gardening and farming to prevent the growth of grass and weeds while permitting moisture to be transmitted through the film to the soil beneath. Perforated film is also used for making disposable diapers, for example, see U.S. Pat. No. 3,814,101.
One of the earlier methods for vacuum perforation of plastic film is disclosed in U.S. Pat. No. 3,054,148. The patentee describes a stationary drum having a molding element or screen mounted around the outer surface of the drum and adapted to freely rotate thereon. A vacuum chamber is employed beneath the screen to create a pressure differential between the respective surfaces of the thermoplastic sheet to be perforated to cause the plasticized sheet to flow into openings provided in the screen and thereby cause a series of openings, holes or perforations to be formed in the plastic sheet or film.
A variety of methods and apparatuses including particular types of perforating screens or rotatable molding elements have been developed over the years for particular perforation operations. Examples of these are U.S. Pat. Nos. 4,155,693, 4,252,516, 3,709,647, 4,151,240, 4,319,868 and U.S. Pat. No. 4,388,056. In U.S. Pat. No. 4,155,693, the screen is comprised of a series of perforated metal strips preferably welded together to form a cylinder. U.S. Pat. No. 4,252,516 provides a screen having a series of hexagonal depressions with elliptical holes centered therein. U.S. Pat. No. 3,709,647 provides for a rotating vacuum-forming roll having a circulating cooling medium therein.
U.S. Pat. No. 4,151,240 provides a means for cooling the film after it has been perforated and debossed. U.S. Pat. No. 4,319,868 sets forth an apparatus for making a thermoplastic film having raised bosses with perforated tips. A particularly constructed embossing roll for effecting the desired film pattern is disclosed. U.S. Pat. No. 4,388,056 discloses an apparatus for continuously forming an air-laid fibrous web having oppositely phased, cylindrically undulating side edges and a predetermined basis weight distribution. An air-laying drum has a honeycomb type annular-shape frame including circumferentially extending ribs and transverse plates. A stationary adjustable air flow modulating means is disposed adjacent the radially inwardly disposed boundary of an arcuate portion of a circumferentially segmented annular-shape plenum, circumferentially spanning a plurality of plenum segments for adjusting a pressure drop across particular areas of the surface of the air-laying drum.
Vacuum perforation of thin plastic films involves the extrusion of molten polymeric materials such as polyethylene and other plastic polymers though a slot die. The hot melt web of film or plastic sheet exiting the die impinges on a rotating cylindrical screen which is mounted on a stationary vacuum drum or roll. The vacuum roll has an axial slot and a set of seals extending longitudinally the length of its outside surface, beneath the area where the web of plastic impinges on the screen or molding element. A high vacuum from inside the screen is directed through the slot in the vacuum roll. The vacuum present within the slot forms or molds the plastic film or sheet to the screen and perforates it through the holes of the screen. At the same time, an airflow is produced which cools the film.
The most important component of the vacuum processing equipment is the cylindrical screen. This molding element defines aesthetic and mechanical properties of the film as well as the geometric pattern of the perforated film. In a preferred screen fabrication technique, the desired screen pattern is nickel plated on a specially prepared cylindrical mandrel. A seamless cylindrical nickel screen of any predetermined or desired pattern can be produced. Other metals, such as copper may also be used.
Some film products require the use of relatively thick screens, i.e., from 0.020 to 0.100 inches thick, and also require that the walls of the patterned holes are straight and perpendicular to the screen surface. Present screen fabrication techniques as heretofore described are not capable of producing a screen meeting these requirements. The patterned holes on screens produced by nickel plating a prepared cylindrical mandrel, even with the application of specialized plating and post etching techniques, take the shape of inverted, truncated, concaved cones. The thicker the screen, the more exaggerated the effect becomes.
It is therefore a principal object of the present invention to provide a method of fabricating relatively thick, seamless, cylindrical metal screens having patterned holes whose walls are substantially straight and perpendicular to the surface of the screen.
Another principal object of the invention is to provide a thick, seamless, cylindrical metal screen which has patterned holes whose walls are substantially straight and perpendicular to the screen surface.
Other objects and advantages of the instant invention will become more readily apparent from the drawings and a reading of the description hereinafter.